In a circular replication system there is currently no mechanism to solve conflicts. So, if two nodes update the same record, the last one overwrites the previous record.
In case of a INSERT of the same record with a UNIQUE key in two nodes, there is going to be a conflict. The replication paradigm can't handle it.
If you were looking for a distributed database where multiple branches update a central repository, replication, with today's technology, is not your solution.
There are plans to introduce conflict resolution mechanisms and multi-master systems in replication, but we are talking about version 5.2 or 5.3.